comparison of synchronous motors with different permanent magnet and winding types

1256 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 3, MARCH 2013

Comparison of Synchronous Motors With DifferentPermanent Magnet and Winding Types

Peter Sekerák , Valéria Hrabovcová , Juha Pyrhönen , Lukáš Kalamen , Pavol Rafajdus , and Matúš Onufer

Faculty of Electrical Engineering, University of Žilina, Žilina 01026, SlovakiaDepartment of Electrical Engineering, Lappeenranta University of Technology, Lappeenranta FI-53851, Finland

This paper deals with the design of permanent magnet synchronous motors (PMSMs) with ferrite magnets and their comparisonwith synchronous machines with NdFeB magnets. In recent years, ferrite magnets have become popular due to their rather low costand, what is more important, due to increasing of the NdFeB magnets cost last time. The progress in ferrite magnet material propertiesduring the latest decade gave rise to their use in high power applications. Their remanent flux density today reaches values beyond 0.4T, which allows design electrical machines with acceptable performance. In this paper, three designs of ferrite machines are presentedand compared with a real PMSM with NdFeB magnets. Furthermore, the influence of the stator winding type and the shape of the slotopening on the PMSM properties will be demonstrated. All machines are designed for the same output power as the original PMSMwith NdFeB magnets. This paper demonstrates that ferrite magnets can, to some extent, be used as replacements for the NdFeB magnetsin high-efficiency power applications.

Index Terms—Concentrated winding, distributed winding, efficiency, ferrite, nonoverlapping winding, permanent magnetsynchronous motors (PMSMs).

I. INTRODUCTION

F ERRITE MAGNETS became popular among the de-signers of electric machines during the latest decade after

the cost of the NdFeB magnets had notably risen. The ferritepermanent magnets (PMs) have been considered for a longtime as the second grade material in comparison with rare earthmagnets because they have a significantly lower remanence, coercivity , and energy product than rare earth

magnets. Today, the progress in ferrite properties and the highcost of NdFeB gives more opportunities for their use. Table Ishows the up-to-date PM properties as they have been gatheredtoday (2012) at the web pages of their producers.AlNiCo magnets have higher remanence than ferrites but,

due to their low coercivity , they can be easily demagnetized.Ferrites, however, have a wide linear demagnetization curveand, therefore, many authors have chosen to study the appli-cability of ferrites in their research concerning the developmentof rotating electrical machines.In [1], Fang et al. investigated two permanent magnet syn-

chronous motors (PMSMs); one with ferrite magnets and an-other one with NdFeB magnets. Both of them were of interiormagnet type with PMs entirely embedded in the rotor. The au-thors have done complete analysis of parameters, losses, andefficiencies of both motors. The rated efficiency of the ferritemotor was about 1% lower than the efficiency in the motor withNdFeB. The volume of ferrites was five times as high as thevolume of NdFeB. The outer dimensions of both motors havebeen the same.In [2], Richter and Neumann have presented two PMSMs,

one with SmCo magnets and another, again, with ferrites. The

Manuscript received July 02, 2012; revised September 21, 2012; acceptedNovember 05, 2012. Date of publication November 29, 2012; date of currentversion February 20, 2013. Corresponding author: P. Rafajdus (e-mail: [email protected]).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TMAG.2012.2230334

TABLE IPM MATERIALS PROPERTIES [5], [6]

efficiency of the ferrite motor was about 0.5% lower at the sameoutput power. The weight of ferrites was 2.5-fold in comparisonwith the weight of SmCo.In [3], Chaudhari and Fernandes have presented a PMSM

with a new rotor design with ferrite magnets and with a damperwinding. The authors compare their machine with another oneat constant ferrite PM volume. The newly designed PMSM has4%-unit improvement in efficiency in comparison with the olddesign.In [4], Jussila et al. investigated the losses in PMSMs with

concentrated nonoverlapping windings and compared themwith losses in a PMSM with distributed windings. All motorshave the same dimensions, air-gap length, and the same amountof PM material. The authors have presented five models ofPMSMs with different amount of slots per pole and phase .The highest efficiency has been achieved with a configurationwith 24 stator slots and 20 rotor poles .In [9], Sekerák et al. have investigated a PMSM with NdFeB

magnets and then the PM material was replaced by ferrites: 1)with the same volume; and 2) with an increased volume to getthe same output power. It was shown that comparable proper-ties have been achieved if greater motor size can be accepted.Therefore, the greater care has been devoted to the ferrite mo-tors, looking for more appropriate design, mainly on the stator.In this paper, not only magnet materials but also the influence

of the stator winding on PMSM properties are investigated. Forthis purpose, three PMSM models are created. Table I lists nec-essary PM data. It will be shown that if NdFeB magnets withproperties close the lower limit of the range and ferrites with

0018-9464/$31.00 © 2012 IEEE

SEKERÁK et al.: COMPARISON OF SYNCHRONOUS MOTORS WITH DIFFERENT PM AND WINDING TYPES 1257

Fig. 1. Cross-sectional area of the original PMSM with its winding arrange-ment.

the values close to upper limits are used, the PMSMs of bothmagnet materials are comparable, if there is no need to keep themachine size constant. The first ferrite PMSM is constructedwith a distributed winding. The second and third models haveconcentrated nonoverlapping windings. The difference betweenthem is in the stator slot shape: The first one is of open slottype and the second one is of semiclosed slot type. All ferritemotor models are compared with a real PMSM with NdFeBmagnets. This motor, in the next designated as original, has beeninvestigated very carefully by various methods and the resultshave been verified by means of measurements. The comparisonhas shown that the employed methods give reliable results and,hence, they can be used in the design procedure, including sim-ulation, for ferrite PMSMs, although no prototypes have beenbuilt until now.

II. ORIGINAL PMSM WITH NDFEB MAGNETS

The real, original PMSM has NdFeB PMs embedded in therotor. This topology is called the interior PMSM. The cross sec-tion of one quarter of the original PMSM is shown in Fig. 1.The nameplate, main parameters, and dimensions of the in-

vestigated original PMSM are shown in Table II.The stator has 48 slots; three of them are empty due to

the stator winding symmetry. The stator winding is of thedistributed fractional slot type.On the base of the cross-sectional area, a 2-D finite-element

model (FEM) has been created for the parameters and propertiesanalysis.The parameters of the equivalent circuits in dq frame (Fig. 2)

have been determined by procedures described in [8] and [9]and are shown in Table III.The values of parameters achieved by different methods have

a good coincidence and the parameter investigation procedurecan be applicable for new ferrite motors.

TABLE IINAMEPLATE, PARAMETERS, AND DIMENSIONS OF THE ORIGINAL PMSM

Fig. 2. Equivalent circuit of the PMSM in the frame.

TABLE IIIPARAMETERS OF THE ORIGINAL PMSM

A. Induced Voltage by PM

is one of the important parameters for the PMSM op-eration. The ratio of the stator phase voltage overis defined. Very high means a big difference be-tween induced and terminal voltage what, depending on the syn-chronous inductance, can result in a high stator current

. This is a base for high Joule losses. Thewaveform of the air-gapmagnetic flux density of the originalPMSM calculated by means of FEM without circuit coupling isdepicted in Fig. 3. Its fundamental harmonic component is0.613 T which is seen in Fig. 4.The induced voltage created in stator winding by PM

was calculated for each th harmonic component by

(1)


Fig. 3. Waveform of the air-gap magnetic flux density of the originalPMSM.

Fig. 4. Harmonic spectrum of the air-gap magnetic flux density of the orig-inal PMSM.

where is frequency, is the number of stator winding turns,is its winding factor, and is the magnetic flux calculated

by

(2)

where is the iron stack effective length, and is the pole pitch.

B. V-Curves

In PMSM, the classical V-curves are not possible due to con-stant excitation by PM. The V-curves in this case can be plottedas the stator current versus ratio of at differentloads.The V-curves can show the optimal operating point in the

lowest point of the V-curve, where the stator current is thesmallest for different loads and voltage levels; see Fig. 7.Operating in this optimal point results in the lowest Joule losses(that represent the majority of all losses) and, hence, in thehighest efficiency.

C. Losses

The iron losses have been calculated by means of the 2-DFEM model from where the waveforms of the magnetic fluxdensities have been obtained; see Fig. 5.Iron losses in the th element (element means tooth, yoke,

etc.) have been calculated by [7]

(3)

where 3.1 Wkg (material M800-65A) is iron loss perunit of mass at magnetic flux density 1 T, is theamplitude of the magnetic flux density in the th element of amachine, and is the mass of the th element of a ma-chine. The investigation was done by means of the 2-D FEM.

Fig. 5. Tangential and normal components of the air-gap magnetic flux densityof the original PMSM at rated load 2 kW and optimal (seeFig. 7).

Fig. 6. waveform of the original PMSM. (a) Simulated by means of FEMand by (1). (b) Measured.

The amplitude of the magnetic flux density in theth-element has two components, tangential and normal ones,from which the total value for the th element is calculated

(4)

D. Verification by Experiments

All investigated parameters and properties of the originalPMSM (NdFeB) have been verified by measurements.Fig. 6(a) shows the simulated induced phase voltage of the

original PMSM by means of FEM and calculated by the sum of(1). In Fig. 6(b), the measured waveform of the phase

voltage of the original PMSM is shown. The coincidence ofmeasured and simulated waveforms is very good which meansthat this procedure is applicable for other PMSMs. The rootmean square (RMS) value of is 132.5 V.Fig. 7 shows the V-curves of the original PMSM where the

simulated data have been verified bymeasurements. The PMSMhas been loaded by loads 1, 1.5, 2, and 2.5 kW. The stator phasevoltage has been changed from 230 to 150 V, which representsthe ratio from 1.73 to 1.13, respectively.The V-curves have shown that the original PMSM has its

optimal working point at at 2 kW. This pointrepresents the stator phase voltage 180 V and 4.78A. The reason for the low terminal voltage is the motor’s lowvalue of 132.5 V in comparison with the stator phasevoltage 230 V which is the rated voltage given in thenameplate.Fig. 7 shows also that at the operating point at the rated

voltage 230 V (ratio 1.73) and rated load 2 kWPMSM works with a high stator current 7.5 A, what


Fig. 7. V-curves of the original PMSM at frequency 36 Hz. Lines representsimulated data, and circles represent measured data.

Fig. 8. Iron losses of the original PMSM; line represents simulated data, andcircles represent measured data.

results in high Joule losses and low efficiency in the originalPMSM. As the optimal point of the original PMSM, we take:

2 kW.The simulated and measured iron losses are shown in Fig. 8.As can be seen in Table III, also all calculated parameters

have been verified by measurements and the coincidence is ac-ceptable. Therefore, all employed methods will be applied fornewly designed PMSMs with ferrite magnets.

III. DESIGN OF NEW PMSMS WITH FERRITES

Three PMSMs with ferrite magnets have been designed. Thegoal of the PMSM design process should be a low-cost motorwhich can develop required output power with excellent effi-ciency. At first, the choice between NdFeB and ferrites has beendone in favor of ferrites. According to [6], the cost per kilogramof NdFeB is at present c. 30 times the cost of ferrite. Accordingto the data in Table I, the values of ferrites close to the upperlimits are half or less of the values of NdFeB close to the lowerlimits. In spite of this fact, in this paper, it will be shown that it ispossible to design a ferrite PMSM with comparable properties.However, the size will be increased because of the increased PMvolume.For the PMSM designs, the ferrite material with the following

parameters in an operating temperature of 75 C was selected:0.45 T, 340 kAm , and 40 kJm . All

the designed PMSMs have rotor surface magnets. The other pa-rameters of the newly designed motors are shown in Table IV.The initial data have been chosen on the base of the original

Fig. 9. Cross-sectional areas of PMSMs with different winding arrangements:(a) motor A; (b) motor B; and (c) motor C.

TABLE IVINITIAL DATA OF PMSM WITH FERRITES

motor. Three motors with ferrites have been designed. At first,there is the motor with a fractional slot distributed winding,called motor A [Fig. 9(a)]. The second motor has nonoverlap-ping concentrated winding and open slot type, and is called


Fig. 10. Waveforms of in: (a) motor A; (b) motor B; (c) motor C; (d) their harmonic components; (e) flux density of the original on the length of PM surfacefor no load condition; (f) flux density of the A, B, and C motors on the length of PM surface for no-load conditions.

TABLE VFERRITE PMSMS EQUIVALENT CIRCUIT PARAMETERS

motor B [Fig. 9(b)]. The third motor has nonoverlapping con-centrated winding and semiclosed slot type, and is called motorC [Fig. 9(c)]. All motors have been designed to achieve ratedparameters as the original motor, namely rated power, speed,and frequency.The magnetic air-gap length is 2 mm, allowing 1 mm for

a magnet retaining ring. Indirect air cooling has been chosen.

Fig. 9 shows the cross-sectional areas of one quarter of all threenewly designed ferrite motors.

IV. SIMULATION OF PMSM OPERATION

The parameters have been determined by means of analyticalcalculations and FEM simulations applied in [8]. These obtainedparameters are in simulations that will be used to compare theproperties of newly designed motors with the original PMSM.The maximal torque, torque ripple, V-curves, losses, and effi-ciency are calculated.Parameters from Table V have been put into the equivalent

circuit model (see Fig. 2) and simulations have been carried out.

A. Air-Gap Magnetic Flux Density and

The air-gap magnetic flux density in all three motors hasbeen investigated by means of 2-D FEM models. Fig. 10 shows


Fig. 11. Calculated maximal developed torque of all motors at the rotor speed360 r/min and rated voltage.

the waveforms of and their harmonic components at no-loadconditions. In Fig. 10(e) and (f), there can be seen the differenceof the flux densities on the length of PM surface for all fourdiscussed motors because of their different stator slot openingand PM position (see Figs. 1 and 9).The fundamental components of are about 0.4 T, which

is very low in comparison with the PMSM with NdFeB mag-nets. NdFeB magnets can provide magnitudes of up to 1 Talthough in real PMSM is lower. Low provided by fer-rites has to be taken into account in a PMSM design and a highernumber of stator turns is required (see Table IV), in comparisonwith in the original PMSM. As a result, the propor-tions of iron and copper in the teeth are changed and in ferritePMSMs more copper material is used. has been calculatedby means of the procedure in Section II-A and the results areshown in Table V.

B. Torque Investigation

Fig. 11 shows the maximal values of the developed torque ofall three ferrite motors and the original one.All three machines are able to develop the rated torque

53 Nm defined by the original PMSM. The highest torquecan be developed by motor B with 90 Nm. On thecontrary, the maximal torque of motor C has been the lowest:

68 Nm. The original PMSM has the highest capabilityto develop the maximal torque 135 Nm.The next investigated parameter is the torque ripple; see

Fig. 12. All three machines are loaded by the rated torque53 Nm.It is seen that motor A has the lowest torque ripple and its

peak-to-peak value is 4 Nm. Motor B has 5.2Nm. The highest value has been found in motor C,8.5 Nm. The original PMSM shows the torque ripple6.5 Nm [12]. The different torque ripple and maximal torqueare given by PMs arrangements and slots opening. In Fig. 12(d),torque ripple harmonic components can be seen. The dominantones are direct current (dc) components around the rated torque.

C. V-Curves

All three motors have been loaded from 1 to 2.5 kW. Thestator terminal phase voltage has been changed in the range from240 to 170 V. The simulated waveforms are shown in Fig. 13. Itis seen that the investigated motors can work very close to theiroptimal point. However, motors A and C do not work in their

Fig. 12. Ripple torques of investigated machines: (a) motor A; (b) motor B; (c)motor C; and (d) their harmonic components.

optimal points [see Fig. 13(a) and (c)], which occurs at highervoltage than 230 V. Therefore, because of windings insulationdimensioned to 230 V, the operating point was designed for thisvoltage. Motor B in Fig. 13(b) works at its optimal condition.

D. Losses and Efficiency

Losses have been investigated in all three motors. The fol-lowing types of losses have been taken into account: Joule losses

, iron losses , and mechanical losses .


Fig. 13. V-curves of the investigated machines: (a) motor A, 211 V;(b) motor B, 188 V; (c) motor C, 213 V.

The losses in the PM have been calculated according to [10]and, due their low values, they can be neglected. The mainreason is the high resistivity of ferrites and, therefore, the eddycurrents and eddy current losses are negligible in ferrite PM.The resistivity of ferrites ( 10 cm) is very high incomparison with the resistivity of NdFeB ( 200cm).The Joule losses are calculated by

(5)

where is the phase resistance at 75 C. The Joule losses arelower for C and B designs, because the concentrated windingsare used with lower phase resistance.Calculation of iron losses has been described in Section II-C

and the same procedure has been applied in ferrite motors.Themechanical loss has been taken into account, and its value

is 40 W for all three ferrite motors. This value hasbeen calculated on the basis of an analytical approach givenin [7]. Mechanical losses of the original PMSM are

Fig. 14. Comparison of losses: 1—motor A; 2—motor B; 3—motor C; and4—original PMSM.

TABLE VICALCULATED EFFICIENCY OF PROPOSED PMSMS

36 W. They were measured and compared with the analyticalapproach.Fig. 14 shows the loss comparison of all three ferrites motors

at rated voltage 230 V and load 2 kW. The losses of the originalPMSM are calculated at decreased phase voltage 180V, 4.78 A. As can be seen from Fig. 14, the losses dependon various factors. If the NdFeB PMs are used, the flux densityis higher and also the iron losses are higher in comparison withthe ferrite PMSM.On the basis of data in Fig. 14, the efficiencies and total losses

of the motors have been calculated and are shown in Table VI.The comparison of the maximal torque, the torque ripple, andthe total material (PM, copper, iron) costs calculated on the baseof actual materials prices is also presented.

V. CONCLUSION

Three motors with ferrite magnets have been designed tocompete with an original motor with NdFeB magnets. Onemotor is with distributed winding and the other two are withconcentrated nonoverlapping windings. It is seen that ferritemagnets can be useful in the PMSM design also when premiumefficiency is required and the properties of the machine withferrites can be comparable with NdFeB machines. A highernumber of stator turns is required in a PMSM with ferrites.That fact can cause a rapid increase of stator leakage andmagnetizing inductance which leads to lower torque capability.The calculations and simulations have shown that semiclosedslot type increases the stator leakage inductance and, therefore,in PMSMs with concentrated nonoverlapping windings, theopen slot type should be used. Although a PMSM with ferritesrequires a big volume of the ferrite PM, which easily results ina bigger size of the motor, on the other hand, the cost of ferritesis lower in comparison with NdFeB. By using a concentratednonoverlapping winding, the iron parts volume has been de-creased in comparison with the original PMSM with NdFeB.


ACKNOWLEDGMENT

This work was supported by the R&D Operational ProgramCentre of Excellence of Power Electronics Systems and Mate-rials for their components II. No. OPVaV-2009/2.1/02-SORO,ITMS 26220120046 funded by the European Regional Devel-opment Fund (ERDF).

REFERENCES

[1] L. Fang, B. H. Lee, J. J. Lee, H. J. Kim, and J.-P. Hong, “Study on high-efficiency characteristics of interior permanent magnet synchronousmotor with different magnet material,” inProc. Int. Conf. Electr. Mach.Syst., 2009, DOI: 10.1109/ICEMS.2009.5382881.

[2] E. Richter and T. Neumann, “Line start permanent magnet motorswith different material,” IEEE Trans. Magn., vol. MAG-20, no. 5, pp.1762–1764, Sep. 1984.

[3] B. N. Chaudhari and B. G. Fernandes, “Synchronous motor usingferrite magnets for general purpose energy efficient drive,” in Proc.TENCON, 1999, vol. 1, pp. 371–374.

[4] H. Jussila, P. Salminen, and J. Pyrhönen, “Losses of a permanentmagnet synchronous motor with concentrated windings,” in Proc. Int.Conf. Power Electron. Mach. Drives, 2006, pp. 207–211.

[5] Materials Magic—Hitachi Metals, “Magnetic appliances,” [Online].Available: www.hitachi-metals.co.jp/e/prod/prod03/p03_10.html

[6] Magsy [Online]. Available: http://www.magsy.cz[7] J. Pyrhonen, T. Jokinen, and V. Hrabovcova, Design of Rotating Elec-

trical Machines. New York, NY, USA: Wiley, 2008.[8] P. Sekerák, V. Hrabovcová, P. Rafajdus, and L. Kalamen, “Interior

permanent magnet synchronous motor parameters identification,” inProc. Int. Symp. Electr. Mach., Prague, Czech Republic, Sep. 2010,pp. 107–116.

[9] P. Sekerák, V. Hrabovcová, L. Kalamen, P. Rafajdus, and M.Onufer, “Synchronous motors with different PM materials,” in Proc.ELEKTRO, May 21–22, 2012, pp. 241–246.

[10] J. Pyrhonen, H. Jussila, Y. Alexandrova, P. Rafajdus, and J. Nerg,“Harmonic loss calculation in rotor surface permanent magnets—Newanalytic approach,” IEEE Trans. Magn., vol. 48, no. 8, pp. 2358–2366,Aug. 2012.

[11] J. Pyrhönen, V. Ruuskanen, J. Nerg, J. Puranen, and H. Jussila, “Per-manent magnet length effects in AC-machines,” IEEE Trans. Magn.,vol. 46, no. 10, pp. 3783–3789, Oct. 2010.

[12] P. Sekerák, V. Hrabovcová, P. Rafajdus, and L. Kalamen, “Emptyslot effect in interior permanent magnet synchronous motor,” in LowVoltage Electr. Mach. Conf., Slapanice, Czech Republic, Nov. 8–9,2010, ISBN: 978-80-214-4178-1, .

Peter Sekerák was born in Stara Lubovna, Slovakia, in 1985. He received theM.Sc. degree in power electrical systems in 2009. Currently, he is working to-

ward the Ph.D. degree at the Department of Power Electrical Systems, Univer-sity of Žilina, Žilina, Slovakia.His research is focused on permanent magnet synchronous machines.

Valéria Hrabovcová graduated in electrical engineering from the University ofŽilina, Žilina, Slovakia, and received the Ph.D. degree in electrical engineeringfrom Slovak University of Technology, Bratislava, Slovakia, in 1985.She is a Professor of Electrical Machines at the Faculty of Electrical

Engineering, University of Žilina. Her professional and research interestsinclude classical, permanent magnets, and electronically commutated electricalmachines.

Juha Pyrhönen received the M.Sc. degree in electrical engineering, the Licen-tiate of Science (technology) degree, and the D.Sc. degree (technology) fromLappeenranta University of Technology (LUT), Lappeenranta, Finland, in 1982,1989, and 1991, respectively.He has served as an Associate Professor in Electric Engineering at LUT

starting in 1993 and was appointed Professor in Electrical Machines and Drivesin 1997. He worked as the Head of the Department of Electrical Engineeringfrom 1998 to 2006. He is active in the research on and development of electricmotors and electric drives.

Lukás̆ Kalamen was born in 1986 in Myjava, Slovakia. After graduating fromthe Faculty of Electrical Engineering, University of Žilina, Žilina, Slovakia, in2009, he received the M.Sc. degree in electrical drives, where he is currentlyworking toward the Ph.D. degree.His main research interest includes electrical machines mainly wind power

systems equipped with induction generators.

Pavol Rafajdus was born in Trnava, Slovakia, in 1971. He received the M.Sc.degree in electrical engineering and the Ph.D. degree from the University ofŽilina, Žilina, Slovakia, in 1995 and 2002, respectively.Currently, he is an Associate Professor at the Faculty of Electrical Engi-

neering, University of Žilina. His research is focused on the electrical machines,mainly switched reluctance motors, and other electrical machine properties.

Matúš Onufer was born in Vranov n. T., Slovakia, in 1987. He graduated fromthe University of Žilina, Žilina, Slovakia, where he received the M.Sc. degreein power electrical engineering in 2011, and currently, he is working toward thePh.D. degree at the Department of Power Electrical Systems.His research is focused on synchronous machines with hybrid excitation.

comparison of synchronous motors with different permanent magnet and winding types

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